C4 is a 200 kDa three-chain glycoprotein which is present in plasma at a concentration of about 350 .mu.g/ml, and which functions as the second complement protein in the classical complement pathway activation sequence.
The binding of an appropriate antibody to a substrate leads to binding and activation of the complex complement protein C1. Activated C1 in turn cleaves a 9 kDa fragment, C4a, from the N-terminal of the C4 alpha chain thereby exposing an internal thio ester which links amino acids at positions 991 and 994 within the C4d region of the C4 alpha' subunit. Upon exposure, this highly reactive group undergoes nucleophilic attack thereby forming a covalent bond with the target substrate.
The major fragment of C4, C4b, is covalently bound to the target substrate following cleavage and release of C4a, and acts as a receptor for C2 of the classical pathway. C2 binds to C4b and is cleaved in turn, by active C1 to continue the complement cascade.
Several proteins in addition to C2 have been identified which bind to C4b. For instance, the C4-binding protein C4-bp, has been found to bind to fluid phase C4b, as well as to C4b which is deposited on a target surface. C4-bp facilitates the cleavage and degradation of C4b which, when cell bound, is cleaved as part of the degradation pathway following association with factor I.
At least two cell membrane-bound proteins also bind to C4b. These proteins include CR1 (the C3b/C4b complement receptor) and gp45-70 which is believed to participate in the degradation of C3b and C4b on cells lacking CR1.
A third cell membrane associated protein also exists which has been designated DAF, decay-accelerating factor. DAF is found on a wide variety of cell types and is a potent inhibitor of C3 convertase of the classical complement pathway and, to a lesser extent, of the alternative complement pathway as well. It is unknown whether this 70 kDa, single chain glycoprotein interacts with C4b/C3b or the enzymatic units C2a or Bb of the C3 convertases.